I have just bought a 2003 Sherco 250 and want to change the engine oil. I have found the drain screw / plug, but would like to how much oil to put back in, what grade (synthetic ?) and where to refill ?

Once drained, put the drain screw/plug back in and lay the bike on its side, so the kickstart lever is on top.

The filler plug is to the top right of the clutch cover.

Fill with 450ml of a 75W gear box oil.

http://www.splatshop.co.uk/oils-lubricants/gear-box-oil/sae-75w.html

The best 75W oil we have used with a Sherco clutch is Putoline Nano Trans:

http://www.splatshop.co.uk/putoline-nano-trans-gp.html